The present disclosure relates to air cavity packages and methods for making the same.
An air cavity package typically includes one or more semiconductor dice attached to a base/flange and surrounded by a dielectric frame with electrical leads bonded on the frame. The dice are electrically joined to the leads with wires, and the package is then sealed with a lid. The air serves as an electrical insulator with a low dielectric constant. Air cavity packages are extensively used for housing high frequency devices such as radio-frequency power transistors and monolithic microwave integrated circuits (MMICs.). Surrounding a high frequency semiconductor chip with air improves the high frequency performance of the chip compared to encapsulation in a material having a higher dielectric constant and loss tangent, (e.g., an over-molding compound such as epoxy). High frequency performance can also be improved by reducing lead inductance between the transistors and leads of the package.
RF device manufacturers desire to minimize material and production costs associated with air cavity packages. Manufacturers have developed backside metallization systems that enable silicon (Si), gallium nitride/silicon carbide (GaN/SiC), or gallium nitride/silicon (GaN/Si) chips to be soldered onto low-cost copper flanges using gold-tin (AuSn) solder or nanosilver. However, it is difficult to braze a dielectric frame to the copper flange and to the electrical leads which satisfies desired cycle properties (e.g., adherence after 1,000 temperature cycles of −65° C. to +150° C.). The dielectric frame is typically made of alumina, but bonding alumina to copper is problematic due to the severe mismatch between the coefficients of thermal expansion (CTEs) of these materials. In particular, the linear CTE of copper is about 17 ppm/° C. at 20° C. whereas the linear CTE of alumina is about 8 ppm/° C. at 20° C. A large alumina frame brazed to a copper flange can only withstand thermal excursions that remain below about 200° C.
Some manufacturers have offered a dielectric frame made of liquid crystal polymer (LCP) which is overmolded onto copper leads to create a frame. LCP has a close CTE match to copper. The frame/lead subassembly can then be bonded onto a copper flange with epoxy after chips have been soldered onto the flange. However, LCP is difficult to bond with epoxy due to its extreme chemical inertness. A common failure mechanism of LCP parts is leakage at the interface between the LCP and a metal (e.g., as observed during gross leak testing in a Fluorinert® bath). Sometimes the flange surface must be roughened in order to achieve adequate adhesion between the flange and the LCP frame. For a number of reasons, the LCP frame is epoxied onto the flange between the steps of die attachment (AuSn or AuSi solder) and wire bonding.
It would be desirable to develop new air cavity packages that are simpler and/or less expensive to produce. It would also be desirable to create an air cavity package with a copper base/flange that is fully assembled with a plastic frame and electrical leads, and that can withstand subsequent assembly operations (e.g., AuSn die attachment and lid attachment) that reach temperatures of 320° C. and can withstand temperature cycling (from −65° C. to +150° C. for one thousand cycles, per MIL-STD-883 Method 1010.8 Condition C).